Niobium complexes cyclopentadienyl

Bis(cyclooctene)—iridium(I) complexes, preparation, 7, 316 Bis(cyclopentadienyl) alkenes, with tantalum, 5, 157 Bis(cyclopentadienyl) alkyne niobium complexes, characteristics, 5, 81... 

Bis(cyclopentadienyl) niobium complexes halides, 5, 90 N-donor ligands, 5, 94 O- and S-donor ligands, 5, 93 P-donor ligands, 5, 94... 

In contrast to the tantalum compounds considered above, the niobium complexes can exist as conformers of different stability at low temperatures. For example, in the H NMR spectrum of complex 4, the intensity ratio between the signals of the conformers is about 3 1 [ 32 ]. The crystal structures of the bis-amido compound 5 and the methylamido compound 6 all show that the lone pair of the amido ligand lies preferentially in the equatorial binding plane of the cyclopentadienyl and imido ligands, a conformation due to the ir bonding interaction of the lone pair with the niobium centre. If rotation about the Nb-Namido bond becomes slow on the NMR time scale then two conformers will be observed, one with the amido proton oriented toward the cyclopentadienyl ring and one with it oriented away. It is supposed that the major conformer is that with the amido proton oriented toward the cyclopentadienyl ring, which minimizes the steric interactions present [32]. 

Insertions of isocyanide into niobium-carbon bonds follow a path similar to that with vanadium, resulting in the formation of the 7]2-iminoacyl complexes, which can then be involved in further chemistry.175 176 The reaction of acetone with cyclopentadienyl complex 110 under a carbon monoxide atmosphere gives the if -acetone compound 111. Complex 111 subsequently undergoes either stepwise insertion of two isocyanides via 112 or double insertion of the isocyanide to give complex 113 (Scheme 48).177... 

In a similar manner, UV irradiation of (triphenylsilyl)phenylacetylene with cyclopentadienylmanganese tricarbonyl 121) and cyclopentadienyl-niobium tetracarbonyl 122) results in elimination of carbon monoxide and formation of monodentate acetylene complexes (R = Ph) ... 

Bis(2,4-pentanedionato)chromium, 3320 Bis(benzene)chromium dichromate, 3845 Bis(benzeneiron)—fulvalenediyl complex, 3828 Bis(cycloctatetraene)uranium(0), 3701 Bis(cyclopentadienyl)bis(pentafluorophenyl)zirconium, 3824 Bis(cyclopentadienyldinitrosylchromium), 3269 Bis(cyclopentadienyl)hexafluoro-2-butynechromium, 3629 Bis(cyclopentadienyl)lead, 3288 Bis(cyclopentadienyl)magnesium, 3271 Bis(cyclopentadienyl)manganese, 3272 Bis(cyclopentadienyl)niobium tetrahydroborate, 3318... 

Cyclopentadienones, in molybdenum carbonyls, 5, 451 Cyclopentadienyl alkenes, with niobium, 5, 76 Cyclopentadienyl-alkoxo complexes, with Ti(IV), 4, 495 Cyclopentadienyl alkyl complexes, with niobium, 5, 66 Cyclopentadienyl-amido complexes, with Ti(IV), via dehalosilylation, 4, 448... 

Cyclopentadienyl aryl complexes, with niobium, 5, 66 Cyclopentadienyl bis(phosphine)amide zirconium compounds, dicarbonyl complexes, 4, 702. -Cyclopentadienyl-carbanionic complexes, with Zr(IV),... 

Cyclopentadienyl carbonyl complexes, with niobium, 5, 64 Cyclopentadienyl chromium bis(Ti-allyl) complexes, with chromium, 5, 305-306 Cyclopentadienyl chromium carbonyls... 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

Mono(cyclopentadienyl) complexes alkenes and tantalum, 5, 157 alkyne and niobium, 5, 80 calcium, strontium, barium, 2, 133... 

Monomeric, paramagnetic chlorohydrides snch as MH2CI2 (PR3)4 derive from facile oxidative addition of H2 to MCl2(PR3)4 complexes. Of these 17e dodecahedral complexes, (32) is the most interesting (eqnation 17). Its thermolysis in solution offers a qnadruply bridged hydride (33) and it can also be rednced to a monomeric Ta 16e species (34). Ta hydrides display a hydridic character (see Hydride Complexes of the Transition Metals), as shown by the reduction of CO by cyclopentadienyl derivatives. (see Niobium Tantalum Organometallic Chemistry)... 

In the solid state at room temperature the compound is stable for only about 1 hr, but it can be stored indefinitely under nitrogen at —20°. The compound should always be handled under an inert atmosphere as it is air-sensitive. Its infrared spectrum consists of strong bands at 1010 and 815 cm-i arising from vibrations of the //-cyclopentadienyl ligand and bands at 1100, 750, 700, and 490 cm-i due to vibrations of coordinated dimethylphenylphosphine. Bands at 1050 and 530 cm i in spectra of the tetrafluoroborate salt or at 830 cm i in those of the hexafluorophosphate salt arise from vibrations of the anion a band at 1740 cm- is due to a niobium-hydrogen stretching vibration. The nmr spectrum (TMS internal standard) at —36° of an acetone-dg solution of the tetrafluoroborate salt is composed of a complex band centered at 72.4, a doublet with Jph = 2.0 Hz centered at t4.42, a doublet with Jph = 7.1 Hz centered at r8.52, and a doublet with Jph = 31.5 Hz centered at t13.96. 

Cyclopropanes can also be obtained in acidolysis reactions of cyclopropene-transition-metal complexes. This reaction has been used analytically to prove the eoordination of an intact three-membered ring to a metal eenter. For example, dichlorobis(> -cyclopentadienyl)niobium (10), upon treatment with sodium amalgam in toluene in the presence of eyclopropene, gives a moss-green eyclopropene eomplex 11 which can be isolated and characterized by its NMR spectrum. Treatment of complex 11 with hydrochloric acid results in the formation of almost pure cyclopropane, according to GC analysis. 

Here, decomplexations of ligands in which the cyclopropane ring is preserved are described. In some cases the ring system can be modified during decomplexation. Thus, the cyclopropene complex 1 obtained via complexation of cyclopropane with bis(cyclopentadienyl)niobium(IV) dichloride under reductive conditions, upon treatment with hydrochloric acid, liberated cyclopropane in > 90% yield. 

Tetrachloro(cyclopentadienyl) complexes of niobium and tantalum, and their ring-substituted derivatives can be prepared by two general, methods. One method uses tin reagents of cyclopentadienes and the other uses trimethylsilyl reagents, which are reacted with niobium or tantulum pentachloride. Here the former method (Method 1) is described for the synthesis of Ta(ii -C5Me5)Cl4, and the latter (Method 2) for the synthesis of Nb(T -C5Me5)Cl4. 

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